Mitigation Option

Costa

Potential Emission Mitigationb (Mt CO2 equivalent/yr)

GEOENGINEERINGc

Low stratospheric sootd

Low

8 × 103 to 25 × 103

Low stratospheric dust-aircraft deliveryd

Low

8 × 103 to 80 × 103

Stratospheric dust (guns or balloon lift)d

Low

+

Cloud stimulated by provision of CCNe

Low

+

Stimulation of ocean biomass with ironf

Low to medium

7 × 103

Stratospheric bubbles (multiple balloons)g

Low to medium

+

Space mirrorsg

Low to medium

+

NOTE: This table summarizes the information by source category and places the options in order of cost-effectiveness for the Category 3 options. Cost-effectiveness estimates are categorized as: Savings (for less than 0), Low (0–$9/t CO2 equivalent), Medium ($10–99/t CO2 equivalent), or High (>$100/t CO2 equivalent). The potential emission savings (which in some cases includes not only the annual emissions, but also changes in atmospheric concentrations already in the atmosphere) for the geoengineering options are also shown.

aCosts are in ranges shown below:

$/t CO2

Savings

<0

Low

0–9

Medium

10–99

High

>100

bThis number assumes that we not only mitigate the impact of current emissions of CO2 and other greenhouse gases, but also the stock of those gases. The CO2-equivalent emission is determined by evaluating the equivalent reduction in radiative forcing (see Appendix Q). These options do not reduce the flow of emissions per se, but rather the impact of greenhouse warming from those emissions. The ''+" indicates that there is no known physical limit to this method assuming these options work as expected, just a limit on the amount of mitigation for which we are willing to pay.

cMitigation options are placed in order of cost-effectiveness based on the average (arithmetic mean) of the costs. Cost ranges are averaged prior to each option addition.

dThese options cannot be considered for use until the possible effects of the soot, dust, or aerosol on the destruction of stratospheric ozone are understood.

eCloud condensation nuclei.

fThis option cannot be considered for use until the possible effects of large-scale iron additions to the ocean biomass are well understood.

gInfeasible options.